This invention is related to methods for the in situ extraction of contaminants from soil, and in particular to such a method using steam injection and vacuum extraction.
Vacuum extraction of volatile organic compounds from soil has been successfully demonstrated and has become a popular remediation technique. See, Bennedsen, Feb. 1987, Pollution Engineering, pp. 66-68; Crow et al., Winter 1987, GWMR, pp. 51-57. The typical approach to vacuum extraction is to install a recovery well in the contaminated region and recover vadose zone gases by applying a sub-atmospheric pressure to the well. The sub-atmospheric pressure is sustained by a blower at the surface. The blower exhausts vapors to the atmosphere or to gaseous phase contaminant recovery equipment such as an activated carbon filter or a combustion chamber, depending on the contaminants or their concentrations.
Contaminant recovery rates depend on the contact between flowing air and the contaminants, as well as local gas concentrations. Local gas concentrations are dependent on diffusion and phase change processes. These, in turn, are dependent on local liquid and gaseous phase concentrations and concentration gradients, vapor pressures of the compounds, and heat transfer. At normal subsurface conditions, recovery rates can be low, which result in long treatment times because of the limiting effects of these mass transfer processes.
Steam heating has been used in the petroleum industry as a method to reduce the viscosity, and thereby increase the mobility, of crude oil in order to enhance its recovery from subsurface reservoirs. In order for steam to flow to recovery wells at the depths petroleum is commonly found, high pressures are required to overcome the hydrostatic pressure caused by the large amount of overlying water. The loss of steam flow control is a problem often encountered when injecting steam into deep reservoirs. The steam tends to migrate upward because of buoyancy effects rather than horizontally into the reservoir. This reduces the volume of the reservoir which is heated.
It is therefore desirable to increase the in situ recovery rates of contaminants in the soil while maintaining control of the recovery process, and to do so in a time--and energy-efficient manner.